The field of the invention is acoustic measuring and more specifically generating an acoustic signal issuing from a compact region in space.
Under certain circumstances, it is desirable to make acoustic measurements with an acoustic "point source". A point source is defined as an infinitesimally small sound source that produces a finite quantity of acoustic power. Usually it is modeled as a pulsating sphere of negligible dimensions producing a finite volume velocity at its surface.
A conventional transducer for generating an acoustic signal from an electrical input is a loudspeaker. Head-Related Transfer Function (HRTF) measurements, which measure the transformation of a sound wave by the head, torso, and ear as it propagates from an external source to the eardrum, have traditionally used conventional loudspeakers, 7 cm or larger in diameter, to generate the acoustic stimulus. At distances of 1 m or more, such loudspeakers are perfectly adequate. At close distances there are serious problems associated with loudspeaker-based measurements. The precise location of a loudspeaker, for example, is not well defined in the near-field. The stimulus is generated by the entire diaphragm of the loudspeaker, and at close distances this may extend over a large region of space: at 12 cm, for example, a 7 cm loudspeaker covers an arc in excess of 30 degrees. The HRTF measured will therefore be, in effect, the average HRTF over the entire region covered by the loudspeaker. Additionally, the directional properties of the loudspeaker may taint the HRTF. When the speaker is near the listener, the high-frequency directionality of the speaker will also cause the sound pressure reaching the head and torso to vary according to the orientation of that region relative to the speaker, This may significantly affect the measured HRTF. While it is possible to build a small loudspeaker, there is a trade-off in loudspeaker design between small size and low-frequency output.
The axial response of a loudspeaker is complicated by its distributed geometry at very close distances. At distances less than 2 a/.lambda., where a is the radius of the loudspeaker and .lambda. is the wavelength of the sound, the intensity along the axis of the loudspeaker does not decrease monotonically with distance, but rather passes through a series of maxima with intervening nulls. For a 15 kHz sound generated by a 7 cm loudspeaker, this effect complicates HRTF measurements at distances less than 10 cm from the surface of the head (approximately 20 cm from the center of the head).
A loudspeaker is also generally large enough to provide a reflective surface when placed sufficiently close to the head. Sound generated by the speaker may be reflected off the head, then be reflected again off the loudspeaker source and back toward the head. These second-order reflections could additionally corrupt a near-field HRTF measurement.
For at least these reasons, an ordinary loudspeaker cannot be used effectively to make near-field HRTF measurements. A key to eliminating the problems associated with loudspeaker measurements is reducing the effective area of the source, i.e., providing an acoustic "point source". Point sources have two important characteristics which cannot be duplicated in any physically realizable acoustic transducers. They radiate sound from a single location in space; and they radiate sound omnidirectionally. Unfortunately, it is impossible to build an infinitesimally small sound transducer with these characteristics. Every realizable transducer has finite dimensions, and therefore generates a positive particle velocity over some finite region of space. The sound pressure generated by such a source at a particular location in space is found by dividing the surface of the source into infinitesimal regions. The contribution of each region is determined by assuming that region is a point source with a certain volume velocity. The surface integral of these contributions over the area of the transducer determines the total signal. In acoustic measurements of the transfer function from a sound source at a particular location in space to a receiver at some other location in space, any measurement with a conventional transducer will in fact be the average transfer function over the region covered by the transducer. In order to control the exact location of a sound source, it is necessary to make the area of the transducer as small as possible.
One possible approach to this problem is the use of small loudspeakers. This would certainly reduce the problems of location, directionality, axial response, and reflections described above. However, due to radiation impedance considerations, there is an inverse relation between the efficiency of a loudspeaker at low frequencies and the size of the loudspeaker. Thus extremely small loudspeakers cannot effectively reproduce wide-band stimuli inclusive of low frequency content.
A second approach to the acoustic point source problem is to generate a wideband stimulus with a relatively large conventional cone loudspeaker and connect this speaker, through an enclosed cavity, to a small diameter metal tube. The sound then propagates down the tube and radiates from the small orifice at the opening of the tube. There are three reasons why this type of system cannot generate low frequency sounds. First, conventional loudspeakers radiate inefficiently when the wavelength of the sound is large relative to the diameter of the speaker, and a conventional cone loudspeaker is simply not powerful enough to produce much output below 1 kHz. Second, it is difficult to prevent low-frequency energy from leaking out of the loudspeaker enclosure. It generally requires massive barriers to prevent the propagation of sound at low frequencies. Such a large and massive source would be unwieldy at best and still would not eliminate the problem of secondary reflections off of the source.
A third way to simulate an acoustic point source consists of a stretched, round membrane which is driven only at its center. If the membrane material is chosen carefully, vibrations propagate down the membrane at the same speed the sound waves propagate in air. This results in a hemispherically symmetrical sound radiation pattern. While this system approximates an acoustic point source, it still apparently requires a round membrane which may reflect scattered sound waves. Also, this system cannot be adapted from commercially available components.
The present invention overcomes problems in the art by generating sound from a compact region of space which is both largely nondirectional at relatively high frequencies and relatively powerful at low frequencies, and is also equipped with an electromagnetic position sensing system that allows accurate measurement of the effective position of the source relative to a reference point.